226 research outputs found
Imaging and Nulling with the Space Interferometry Mission
We present numerical simulations for a possible synthesis imaging mode of the
Space Interferometer Mission (SIM). We summarize the general techniques that
SIM offers to perform imaging of high surface brightness sources, and discuss
their strengths and weaknesses. We describe an interactive software package
that is used to provide realistic, photometrically correct estimates of SIM
performance for various classes of astronomical objects. In particular, we
simulate the cases of gaseous disks around black holes in the nuclei of
galaxies, and zodiacal dust disks around young stellar objects. Regarding the
first, we show that a Keplerian velocity gradient of the line-emitting gaseous
disk -- and thus the mass of the putative black hole -- can be determined with
SIM to unprecedented accuracy in about 5 hours of integration time for objects
with H_alpha surface brigthness comparable to the prototype M 87. Detections
and observations of exo-zodiacal dust disks depend critically on the disk
properties and the nulling capabilities of SIM. Systems with similar disk size
and at least one tenth of the dust content of beta Pic can be detected by SIM
at distances between 100 pc and a few kpc, if a nulling efficiency of 1/10000
is achieved. Possible inner clear regions indicative of the presence of massive
planets can also be detected and imaged. On the other hand, exo-zodiacal disks
with properties more similar to the solar system will not be found in
reasonable integration times with SIM.Comment: 28 pages, incl. 8 postscript figures, excl. 10 gif-figures Submitted
to Ap
Consumer-Directed Health Insurance vs. Managed Care: Analysis of Health Care Utilization and Expenditure Incurred by Employees in a Rural Area
Consumer-Directed Health Plans (CDHPs) are proposed as an option to control healthcare costs. No research has addressed their applicability in rural settings. This study analyzes three years (2003–2005) of healthcare expenditure and utilization incurred by two employers and a national carrier providing data from a rural state, Kentucky. The study included two measures of expenditures (health care and prescription drugs) and three measures of utilization (physician visits, hospital admissions, and hospital inpatient days). In general, the CDHP successfully controlled the growth of medical costs. These findings suggest that CDHPs may be a viable alternative benefit structure for rural employers
Odd-Order Aberration-Cancellation in Correlated-Photon Imaging
We discuss a correlated two-photon imaging apparatus that is capable of
producing images that are free of the effects of odd-order aberration
introduced by the optical system. We show that both quantum-entangled and
classically correlated light sources are capable of producing the desired
spatial-aberration cancelation
A single-column model intercomparison of a heavily drizzling stratocumulus-topped boundary layer
This study presents an intercomparison of single-column model simulations of a nocturnal heavily drizzling marine stratocumulus-topped boundary layer. Initial conditions and forcings are based on nocturnal flight observations off the coast of California during the DYCOMS-II field experiment. Differences in turbulent and microphysical parameterizations between models were isolated by slightly idealizing and standardizing the specification of surface and radiative fluxes. For most participating models, the case was run at both typical operational vertical resolution of about 100 m and also at high vertical resolution of about 10 m. As in prior stratocumulus intercomparisons, the simulations quickly develop considerable scatter in liquid water path (LWP) between models. However, the simulated dependence of cloud base drizzle fluxes on LWP in most models is broadly consistent with recent observations. Sensitivity tests with drizzle turned off show that drizzle substantially decreases LWP for many models. The sensitivity of entrainment rate to drizzle is more muted. Simulated LWP and entrainment are also sensitive to the inclusion of cloud droplet sedimentation. Many models underestimate the fraction of drizzle that evaporates below cloud base, which may distort the simulated feedbacks of drizzle on turbulence, entrainment, and LWP
The PreVOCA experiment: modeling the lower troposphere in the Southeast Pacific
The Preliminary VOCALS Model Assessment (PreVOCA) aims to assess contemporary atmospheric modeling of the subtropical South East Pacific, with a particular focus on the clouds and the marine boundary layer (MBL). Models results from fourteen modeling centers were collected including operational forecast models, regional models, and global climate models for the month of October 2006. Forecast models and global climate models produced daily forecasts, while most regional models were run continuously during the study period, initialized and forced at the boundaries with global model analyses. Results are compared in the region from 40° S to the equator and from 110° W to 70° W, corresponding to the Pacific coast of South America. Mean-monthly model surface winds agree well with QuikSCAT observed winds and models agree fairly well on mean weak large-scale subsidence in the region next to the coast. However they have greatly differing geographic patterns of mean cloud fraction with only a few models agreeing well with MODIS observations. Most models also underestimate the MBL depth by several hundred meters in the eastern part of the study region. The diurnal cycle of liquid water path is underestimated by most models at the 85° W 20° S stratus buoy site compared with satellite, consistent with previous modeling studies. The low cloud fraction is also underestimated during all parts of the diurnal cycle compared to surface-based climatologies. Most models qualitatively capture the MBL deepening around 15 October 2006 at the stratus buoy, associated with colder air at 700 hPa
Predictive use of the Maximum Entropy Production principle for Past and Present Climates
In this paper, we show how the MEP hypothesis may be used to build simple
climate models without representing explicitly the energy transport by the
atmosphere. The purpose is twofold. First, we assess the performance of the MEP
hypothesis by comparing a simple model with minimal input data to a complex,
state-of-the-art General Circulation Model. Next, we show how to improve the
realism of MEP climate models by including climate feedbacks, focusing on the
case of the water-vapour feedback. We also discuss the dependence of the
entropy production rate and predicted surface temperature on the resolution of
the model
Astrometric Methods and Instrumentation to Identify and Characterize Extrasolar Planets: A Review
I present a review of astrometric techniques and instrumentation utilized to
search for, detect, and characterize extra-solar planets. First, I briefly
summarize the properties of the present-day sample of extrasolar planets, in
connection with predictions from theoretical models of planet formation and
evolution. Next, the generic approach to planet detection with astrometry is
described, with significant discussion of a variety of technical, statistical,
and astrophysical issues to be faced by future ground-based as well as
space-borne efforts in order to achieve the required degree of measurement
precision. After a brief summary of past and present efforts to detect planets
via milli-arcsecond astrometry, I then discuss the planet-finding capabilities
of future astrometric observatories aiming at micro-arcsecond precision.
Lastly, I outline a number experiments that can be conducted by means of
high-precision astrometry during the next decade, to illustrate its potential
for important contributions to planetary science, in comparison with other
indirect and direct methods for the detection and characterization of planetary
systems.Comment: 61 pages, 8 figures, PASP, accepted (October 2005 issue
Submillimetric GPS distance measurement over short baselines: case study in inner consistency
Distance determination in the open air with submillimetric accuracy is a challenging task
usually carried out with the use of submillimetric distancemeters and costly observation
campaigns. The present paper represents a first step in the research of the potential use of GPS
for submillimetric distance determination for distances up to a few hundred metres consisting
in the evaluation of GPS distance determination reproducibility. As will be concluded, reliable
submillimetric precision is attainable after some hours of observation if the same equipment in
both baseline ends is used, even considering that there still remain some long-term systematic
effects of a few tenths of a millimetre. The need for precise absolute antenna calibration values
is also shown to be critical for submillimetric distance reproducibility.This research is funded by the Spanish Ministry of Science and Innovation (AYA2011-23232). The authors are grateful to the editor and the anonymous reviewers for their valuable suggestions, corrections and comments that helped improve the original manuscript.Baselga Moreno, S.; García-Asenjo Villamayor, L.; Garrigues Talens, P. (2013). Submillimetric GPS distance measurement over short baselines: case study in inner consistency. Measurement Science and Technology. 24(7):750011-750018. https://doi.org/10.1088/0957-0233/24/7/075001S750011750018247Amiri-Simkooei, A. R., & Tiberius, C. C. J. M. (2006). Assessing receiver noise using GPS short baseline time series. GPS Solutions, 11(1), 21-35. doi:10.1007/s10291-006-0026-8Bruyninx, C., Altamimi, Z., Boucher, C., Brockmann, E., Caporali, A., Gurtner, W., … Weber, G. (2009). The European Reference Frame: Maintenance and Products. International Association of Geodesy Symposia, 131-136. doi:10.1007/978-3-642-00860-3_20Doloca, N. R., Meiners-Hagen, K., Wedde, M., Pollinger, F., & Abou-Zeid, A. (2010). Absolute distance measurement system using a femtosecond laser as a modulator. Measurement Science and Technology, 21(11), 115302. doi:10.1088/0957-0233/21/11/115302Dow, J. M., Neilan, R. E., & Rizos, C. (2009). The International GNSS Service in a changing landscape of Global Navigation Satellite Systems. Journal of Geodesy, 83(3-4), 191-198. doi:10.1007/s00190-008-0300-3Firuzabadì, D., & King, R. W. (2011). GPS precision as a function of session duration and reference frame using multi-point software. GPS Solutions, 16(2), 191-196. doi:10.1007/s10291-011-0218-8Hyun, S., Kim, Y.-J., Kim, Y., Jin, J., & Kim, S.-W. (2009). Absolute length measurement with the frequency comb of a femtosecond laser. Measurement Science and Technology, 20(9), 095302. doi:10.1088/0957-0233/20/9/095302Koivula, H., Häkli, P., Jokela, J., Buga, A., & Putrimas, R. (2011). GPS Metrology: Bringing Traceable Scale to a Local Crustal Deformation GPS Network. International Association of Geodesy Symposia, 105-112. doi:10.1007/978-3-642-20338-1_13Ray, J., Altamimi, Z., Collilieux, X., & van Dam, T. (2007). Anomalous harmonics in the spectra of GPS position estimates. GPS Solutions, 12(1), 55-64. doi:10.1007/s10291-007-0067-7Schuhler, N., Salvadé, Y., Lévêque, S., Dändliker, R., & Holzwarth, R. (2006). Frequency-comb-referenced two-wavelength source for absolute distance measurement. Optics Letters, 31(21), 3101. doi:10.1364/ol.31.003101Snay, R. A., & Soler, T. (2008). Continuously Operating Reference Station (CORS): History, Applications, and Future Enhancements. Journal of Surveying Engineering, 134(4), 95-104. doi:10.1061/(asce)0733-9453(2008)134:4(95
Femto-photography: capturing and visualizing the propagation of light
We present femto-photography, a novel imaging technique to capture and visualize the propagation of light. With an effective exposure time of 1.85 picoseconds (ps) per frame, we reconstruct movies of ultrafast events at an equivalent resolution of about one half trillion frames per second. Because cameras with this shutter speed do not exist, we re-purpose modern imaging hardware to record an ensemble average of repeatable events that are synchronized to a streak sensor, in which the time of arrival of light from the scene is coded in one of the sensor's spatial dimensions. We introduce reconstruction methods that allow us to visualize the propagation of femtosecond light pulses through macroscopic scenes; at such fast resolution, we must consider the notion of time-unwarping between the camera's and the world's space-time coordinate systems to take into account effects associated with the finite speed of light. We apply our femto-photography technique to visualizations of very different scenes, which allow us to observe the rich dynamics of time-resolved light transport effects, including scattering, specular reflections, diffuse interreflections, diffraction, caustics, and subsurface scattering. Our work has potential applications in artistic, educational, and scientific visualizations; industrial imaging to analyze material properties; and medical imaging to reconstruct subsurface elements. In addition, our time-resolved technique may motivate new forms of computational photography.MIT Media Lab ConsortiumLincoln LaboratoryMassachusetts Institute of Technology. Institute for Soldier NanotechnologiesAlfred P. Sloan Foundation (Research Fellowship)United States. Defense Advanced Research Projects Agency (Young Faculty Award
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